From U.S. Pat. No. 4,665,732 a device is known for cold expansion of holes by means of a tool comprising a longitudinally split hollow mandrel. A round pin is introduced into the axial hole of the mandrel. The device comprises a hydraulic cylinder in which a first piston with a piston rod is placed to whose end the longitudinal split hollow mandrel of the tool is fixed. A support in contact with the work piece to be worked during pulling the hollow mandrel through the hole, is fixed immovably. In an axial hole worked in the first piston and its piston rod, a second piston rod is inserted whose end is connected to the round pin which makes the mandrel rigid. A second piston connected to the second piston rod is placed in a second cylinder coaxial to the first one and connected to it. The device has a power hydraulic system driven and controlled by means of compressed air.
When working fastener holes using the known device and tool the degree of cold expansion of the hole depends only on the diameter dimensions of the mandrel and pin as well as on the diameter of the previously drilled hole. To guarantee the degree of cold expansion specified with a certain tolerance, it is necessary to control by geometric criterion by means of gauges both the diameter of the previously drilled hole and the mandrel working part.
Furthermore, working is carried out sequentially, expansion is performed layer by layer along the hole axis, whereupon a deformation wave passes causing at every point of the hole surface alternating stresses whose absolute value exceeds significantly the yield limit of the material. Due to a support placed against the movement of the stiffened mandrel, an axial force flow passes through the work piece being worked and as a result an asymmetric axial gradient of the generated zone of compression residual stresses around the hole is obtained. At the hole entrance residual stresses with minimum absolute value occur. Furthermore, at the entrance and exit, around the hole edge, significant axial deformations appear known as “surface upset”
As a result of all this, when employing the known device and tool for cold extension, along the axis of the hole to be worked, a non-uniform and asymmetric zone of compression residual stresses arises, which is a prerequisite for fatigue cracks growth, initiating from the entrance face with residual stresses with minimum absolute value. The axial deformation along the hole edges are a prerequisite for wear and growth of microcracks in consequence of the detrimental fretting effect. As a result, cyclically or dynamically loaded structural elements will have shorter fatigue life and respectively less reliability. In case of static loads, the resistance of the elements to crack appearance will be lowered.
From BG Patent No 66052 a tool for cold expansion of holes is known, comprising a mandrel with a conical working part. Between the mandrel and the hole to be worked is positioned a longitudinally split into three, four or more portions cylindrical sleeve with a conical hole at one end which has a cone angle α equal to the angle of the working conical part of the mandrel. The longitudinally split cylindrically shaped sleeve comprises an immovably fixed non-split part in its other end which is ⅕ of the total length of the sleeve. As a result of the relative axial displacement between the mandrel and sleeve, its split part enhances its radial dimensions, expanding in this way the hole being worked, on the account of inserting the conical working part of the mandrel into the conical hole of the sleeve.
When expanding fastener holes with the known tool, there is an edge contact between the mandrel conical surface and the conical surfaces of the working parts of the sleeve during their relative motion along their common axis because circular cross sections with a larger radius get into contact with the cross sections of the working parts of the sleeve having a smaller radius. In this way, in the process of working the hole, the mandrel conical surface is in contact with longitudinally oriented edges of the working parts of the split sleeve. As a result, the transfer function between the axial displacement of the mandrel and the radial displacement of the working parts of the split sleeve is non-linear, and therefore practically it is not possible to control a precisely preset degree of cold expansion of the hole to be worked. At the same time, this edge contact leads to useless increase in the axial force applied to the mandrel, required for working the hole. Furthermore, the edge contact parameters vary at each instant of the mandrel motion and hence the driving device is subjected to undesired pulsations. At the initial moment of the cold expansion process when the working parts of the sleeve are positioned in the hole to be worked, between every two working parts there is a clearance equal to the thickness of the tool with which the sleeve was split. In the process of cold expansion this clearance increases. As a result, in the cross section of the hole, the zone of compression residual stresses, though it has a pole of symmetry, it is non-homogeneous. Furthermore, the requirement that the outer cylindrical surface of each working part should have a radius larger than the radius of the previously drilled hole that is to be expanded, causes manufacturing difficulties when making the sleeve, due to the fact that it is not entirely split.